This invention relates to an apparatus and method for improving the colour spectrum of the light emitted from a high intensity discharge (HID) lamp when operating under less than nominal power conditions (i.e. when dimmed). The invention is particularly, though not exclusively, suited to the ballasting of metal halide discharge lamps. Typically such systems can be used for highway lighting, architectural floodlighting, warehouse, retail display and industrial lighting.
Traditionally, ballasting for HID lamps is by use of inductors or chokes capable of controlling the lamp current through the impedance they present in series with the mains supply voltage. With some types of HID lamp a high striking voltage, typically 4-5 kV, is required to ionize the gas filling the tube and to initiate the arc.
In prior art systems for ballasting HID lamps, the lamp ballasting means and the lamp striking means are typically discrete circuit elements. Historically, HID lamps have been ballasted by using the impedance of a series connected inductor for controlling the lamp current and a separate starter or igniter module to provide the necessary high voltage to strike the lamp.
More recently electronic ballasting means have been devised. Referring to FIGS. 1 and 2, a conventional power factor controller 2 is formed by transistor TR1, inductor L1, diode D1 and capacitor C1. Alternating positive and negative output voltage is provided to the lamp by a full bridge arrangement of a lamp bridge 4 comprising four transistors TR3,TR4,TR5,TR6. The transistors are alternately switched on and off in complementary pairs TR3,TR6 and TR4,TR5 at a low frequency, typically 100-200 Hz.
Connected in series with the lamp across the bridge is an igniter circuit 5 comprising pulse transformer TX1, a Sidac, capacitor C3 and resistor R1. When the igniter circuit operates, the capacitor C3 charges through resistor R1 to a voltage at which the Sidac device switches on, discharging the capacitor C3 into the primary winding of the transformer TX1. The voltage applied to the transformer primary is multiplied by the high turns ratio of the transformer and is sufficient to ionize the gas filling the lamp""s arc tube, thereby initiating an arc.
Since the voltage is AC, the arc will be extinguished when the lamp current approaches zero and the voltage applied to the tube is subsequently reversed. Therefore the igniter must operate again in the opposite voltage half cycle to re-strike the arc for the flow of current in the opposite direction. This ignition cycle is repeated until the lamp electrodes are sufficiently heated by the arc current for thermionic emission to take place. Then the arc voltage in the tube falls below the threshold voltage of the Sidac and arc current is maintained without operation of the igniter circuit.
A further transistor TR2 of a current source 3 controls the flow of current in the output bridge circuit and consequently controls the lamp current. Transistor TR2 is turned on until the current in inductor L2 reaches a preset threshold value, then the transistor TR2 is turned off. Current continues to flow via a diode D2 until the current has decayed to another preset threshold value, then the transistor TR2 is turned on again. Typically this controlled current is further controlled by a lamp power control circuit 6 which takes account of lamp arc voltage and sets the lamp current accordingly so as to maintain the lamp power (watts) at a near constant value over a range of lamp arc voltages.
Thus the lamp current is substantially square wave with a small amount of high frequency ripple current superimposed, caused by the switching of TR2 and the consequent high frequency current modulation caused thereby. FIG. 3 illustrates schematically a typical lamp current waveform in accordance with the above prior art, with the lamp operating at nominal power.
Recently electronic ballasts for metal halide lamps were introduced with the capability for dimming the lamp to approximately half power. This was done by reducing the square wave lamp current amplitude (under the control of a lamp power control circuit 6) so as to reduce the lamp power to a desired value. Such means whilst effective in controlling the lamp power and consequent light output to a desired reduced value are deficient in maintaining desirable colour rendition from the lamp. This is due to the lamp operating at reduced temperature and some of the various elements used in the filling (Dose) of the lamp arc tube condensing out in the arc tube. Typically the operation of a metal halide discharge lamp at reduced power results in a relative reduction in light output near the red end of the colour spectrum and a relative increase in the light output at the blue/green end of the colour spectrum. Such colour shift may be acceptable in some lighting applications but is not acceptable in particular in retail display lighting applications and other applications where the perceived colour in the lit area is important to sales or safety. FIG. 4 illustrates a typical lamp current waveform in accordance with the prior art with the lamp operating at a dimmed or reduced power.
The invention provides a circuit for improving the light spectrum emitted from a dimmed high intensity discharge lamp by superimposing a high frequency waveform onto a substantially square wave lamp current derived from a high intensity discharge lamp ballast, characterized in that the frequency and mark to space ratio of the high frequency waveform are such that for each half cycle of a resultant waveform, the magnitude of a RMS lamp current is greater than the magnitude of an average lamp current.
The invention further provides a method for improving the light spectrum emitted from a dimmed high intensity discharge lamp wherein a high frequency waveform is superimposed on a substantially square wave lamp current derived from a high intensity discharge lamp ballast, characterized in that the frequency and mark to space ratio of the high frequency waveform are such that for each half cycle of a resultant waveform, the magnitude of a RMS lamp current is greater than the magnitude of an average lamp current.
Within both aspects of the invention, the modulation of lamp arc current increases the RMS value of the lamp arc current whilst maintaining an average lamp arc current commensurate with dimmed or reduced power operation. Substantially the average value of light output from a typical metal halide lamp or other high intensity discharge lamp is controlled by the average current whilst, substantially the operating temperature of a typical metal halide lamp or other high intensity discharge lamp arc tube and contents thereof is controlled by the RMS arc current. Thus by increasing the RMS content of the lamp arc current whilst maintaining the average current at a value commensurate with dimmed operation an increase in arc tube and contents thereof operating temperature may be affected. This increased operating temperature of the arc tube and contents keeps the dose elements in the gaseous state and improves the colour rendering of the lamp nearer to that which is expected of a lamp operating at nominal power.